The present invention relates to subterranean operations, and more particularly, to settable fluids comprising vitrified shale and hydrated lime, and methods of using such settable fluids in subterranean applications.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be circulated through a drill pipe and drill bit into the well bore, and subsequently flow upward through the well bore to the surface. The drilling fluid functions, inter alia, to cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by providing hydrostatic pressure to counteract the sudden entrance into the well bore of high-pressure formation fluids, and remove drilled cuttings from the well bore. While drilling fluids are generally not settable, e.g., they do not set into hard impermeable sealing masses when static, drilling fluids may increase in gel strength over time. Typically, after a well bore is drilled to a desired final depth, the drill pipe and drill bit are withdrawn from the well bore and the drilling fluid is left therein so as to, inter alia, provide hydrostatic pressure on permeable formations penetrated by the well bore, thereby preventing the flow of formation fluids into the well bore.
A common subsequent step in completing the well bore usually involves placing a pipe string, e.g., casing, into the well bore. Depending upon factors such as, inter alia, the depth of the well bore and any difficulties in placing the pipe string therein, the drilling fluid may remain relatively static in the well bore for an extended period of time, e.g., up to 2 weeks. During that time, the drilling fluid may progressively increase in gel strength, whereby portions of the drilling fluid in the well bore may become increasingly difficult to displace.
Upon placement of the pipe string in the well bore, primary cementing is typically performed, whereby the pipe string disposed in the well bore is cemented by pumping a cement composition through the pipe string and into an annulus between the pipe string and the walls of the well bore, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength during its residence within the well bore, an operator may be unable to displace all of the drilling fluid with the cement composition. Accordingly, portions of the drilling fluid in the well bore may be bypassed by the cement composition. This is problematic, because the drilling fluid generally is not settable; therefore, formation fluids may enter and flow along the well bore, which generally is highly undesirable.
Operators have attempted to solve this problem by developing settable fluid compositions, inter alia, to displace drilling fluids from well bores promptly after their use. However, these methods generally have not met with success, as conventional settable fluids include blast furnace slag and other hydraulic components that may begin to set at relatively low temperatures, e.g., temperatures less than about 90° F. Also, certain slag-containing settable fluids may be intolerant to cement composition contamination, causing the settable fluids to cement prematurely upon contact with well cement. To counteract this tendency to prematurely set, oftentimes a strong set retarder will be added to the displacement fluid, and the displacement fluid often is separated from the cement composition by a spacer fluid. Furthermore, certain blast furnace slags may have varying compositions and physical properties (e.g., particle size) depending not only upon the supplier of a given batch of blast furnace slag, but also upon the particular furnace and process that produced the batch. This may adversely affect the properties of the settable fluid comprising the blast furnace slag.